Reduction of susceptibility artifacts in subencoded single-shot magnetic resonance imaging

ABSTRACT

In a magnetic resonance imaging method an echo train is generated of successive magnetic resonance signals from an object to be examined. The magnetic resonance signals are received with a degree of undersampling and by means of a receiver antennae system having a spatial sensitivity profile and the degree of undersampling is set on the basis of an amount of phase evolution due to a magnetic susceptibility distribution of the object to be examined.

BACKGROUND

The invention relates to a magnetic resonance imaging method comprising

-   -   applying a static main magnetic field with a main field strength        and acquisition sequence with an RF-excitation which generates        an echo train of successive magnetic resonance signals from an        object to be examined    -   receiving the magnetic resonance signals with a degree of        undersampling and by means of a receiver antennae system having        a spatial sensitivity profile and    -   reconstructing a magnetic resonance image from the magnetic        resonance signals and the spatial sensitivity profile.

Such a magnetic resonance imaging method is usually indicated as aparallel imaging method and is known from the paper by K. Pruessmann et.al. in Magn. Reson. Med. 42(1999)952-962.

The known method is in particular known as the SENSE-technique. Theundersampling of the magnetic resonance signals is associated withundersampling in the k-space and reduces the time required for scanningthe k-space. However, parallel imaging methods generate magneticresonance signals that intrinsically have a relative low signal-to-noiseratio (SNR). In particular, the SNR decreases with increasing degree ofundersampling.

SUMMARY

An object of the invention is to optimise the degree of undersamplingsuch that the magnetic resonance image has a high diagnostic quality andscanning of k-space is completed in a relatively short time.

This object is achieved by the magnetic resonance imaging methodaccording to the invention, wherein the degree of undersampling is seton the basis of an amount of phase evolution due to a magneticsusceptibility distribution of the object to be examined.

The present invention is based on the insight that the diagnosticquality of the magnetic resonance image may be compromised bysusceptibility artefacts. Owing to a variance of the magneticsusceptibility in the object to be examined, excited spins in the objectacquire varying phase errors during the echo train. Accordingly, themagnetic resonance signals, in particular in the form of gradientechoes, in the echo train incur phase errors that are larger for laterechoes in the train. In particular, in diffusion weighted imaging a spinecho is generated which is followed by a series of gradient echos,notably these gradient echoes are sensitive for phase errors, while thespin echo is to some extent inherently corrected for phase errors.According to the invention, the degree of undersampling is chosen sosmall as to achieve a sufficiently low degree of undersampling so thatthe SNR is sufficiently high and on the other hand, the degree ofundersampling is chosen sufficiently large so that the time for scanningk-space, and accordingly the duration of the echo train is sufficientlysmall to avoid serious phase errors to be incurred in the later portionof the echo train.

In particular the method of the invention is advantageously implementedin the form of a single-shot EPI(ssbEPI) sequence. The undersamplingallows a relevant portion of k-space to be scanned using only a singleRF-excitation to generate the echo train. Consequently, phase navigatorgating can be avoided. Good results are achieved by employing the methodof the invention to functional MRI(fMRI), such as in the context ofblood oxygenation level dependent (BOLD) fMRI. To allow the BOLD effectto develop, the echo time of the magnetic resonance signals is set in arange of 30-40 ms. Particularly good results are achieved by employing ahalf-Fourier magnetic resonance signal acquisition which starts rightafter the RF-excitation pulse and reaches the centre of k-space at theset echo time.

These and other aspects of the invention will be elaborated withreference to the preferred implementations as defined in the dependentClaims.

Preferably, the degree of undersampling is chosen in dependence of themain field strength. Artefacts due to phase errors caused bysusceptibility artefacts tend to increase with higher main fieldsstrength in the range of 1.5 T to 3.0 T, 7.0 T or even higher. Accordingto the invention the degree of undersampling is chosen such that on theone hand the echo train length is limited so as to avoid substantialphase errors and on the other hand the degree of undersampling isrelatively low so that the SNR is sufficient to reconstruct the magneticresonance image with a high diagnostic quality. Further, it has beenfound that the contrast-to-noise ratio of the magnetic resonance imageincreases at higher main field strength, e.g. at 3 T, which counteractsthe drawback of decrease of SNR due to undersampling.

Preferably, the present invention is employed in diffusion weighted ordiffusion tensor magnetic resonance imaging techniques. To that end theacquisition sequence includes a diffusion sensitisation sub-sequence.Such a diffusion sensitisation sub-sequence includes for example abipolar gradient pulse pair or a gradient pulse pair separated by arefocusing RF pulse. Preferably, the diffusion sensitisation isimplemented in an orientation dependent way such that the diffusiontensor elements can be derived from the magnetic resonance signals. Thisimplementation of the invention notably allows non-invasive tracking ofcortical white matter tracts in the human brain.

Parallel imaging, in particular the SENSE technique is advantageous inhigh-resolution fMRI, notably because the reduction of the signalacquisition time leads to a reduction in in-plane susceptibilityartefacts. However, compared to fall sampling, such as full Fourierencoding, parallel imaging leads to a reduction of the signal-to-noiseratio of the fMRI images. It appears that an optimum compromise betweenartefact reduction and maintaining an adequate signal-to-noise level isachieved at a SENSE reduction factor R of about R=2, in particular thisoptimum holds for fMRI studies in medial temporal brain regions.

The present invention achieves a spatial resolution of the diffusionweighted or diffusion tensor images in the sub-millimetre region,notably for a main field strength of about 3 T. Application of parallelimaging, such as the SENSE technique, effectively reduces susceptibilityartefacts and blurring. Further, the signal-to noise ratio of theundersampled magnetic resonance signals is effectiely enhanced in anoptimum range of the SENSE reduction factor R, which represents thedegree of undersampling. At low R, the signal-to-noise ratio increasesas the number of sampled k-space profiles decreases. This is due to themitigation of T2-decay by faster k-space sampling. At higher values ofR, the signal-to-noise ratio decreases as a consequence of noiseenhancement due to ill-conditioning of the reconstruction procedure;this decrease of the signal-to-noise is often represented by a so-calledgeometry factor.

This signal-to-noise notably decreases due to enhanced noise correlationof inductive coupled receiver antennae. In practice the optimum range ofthe reduction factor It is about [1.9-3.3]. The optimum value is furtherdependent on a degree of partial Fourier acquisition. Good results areparticularly achieved when R=2.4 is combined with 60% partial Fourieracquisition. For single-shot spin echo EPI (sabSE-EPI) with a 256×256matrix, a main field strength of 3 T a scan time as short as 3.5 minswas achieved.

Preferably, the magnetic resonance images are repeatedly acquired, forexample each of the magnetic resonance signals are acquired four timesand a series of repeated magnetic resonance images are reconstructed.The magnetic resonance signals are ‘gated’, i.e. only the magneticresonance image that can be reliably matched are actually averaged. Forexample, only pairs of magnetic resonance images which match better thana predetermined critical match value are taken into account foraveraging. Consequently, the averaging increases the SNR of the averagedimage while due to the ‘gating’ hardly any or no artefacts areintroduced.

The time required for acquisition of the magnetic resonance (MR) signalsis reduced by employing sub-sampling of the MR-signals. Suchsub-sampling involves a reduction in k-space of the number of sampledpoints which can be achieved in various ways. Notably, the MR signalsare picked-up through signal channels pertaining to several receiverantennae, such as receiver coils, preferably surface coils. Acquisitionthrough several signal channels enables parallel acquisition of signalsso as to further reduce the signal acquisition time.

Owing to the sub-sampling, sampled data contain contributions fromseveral positions in the object being imaged. The magnetic resonanceimage is reconstructed from the sub-sampled MR-signals with the use of asensitivity profile associated with the signal channels. Notably, thesensitivity profile is for example the spatial sensitivity profile ofthe receiver antennae, such as receiver coils. Preferably, surface coilsare employed as the receiver antennae. The reconstructed magneticresonance image may be considered as being composed of a large number ofspatial harmonic components which are associated withbrightness/contrast variations at respective wavelengths. The resolutionof the magnetic resonance image is determined by the smallestwavelength, that is by the highest wavenumber (k-value). The largestwavelength, i.e. the smallest wavenumber, involved, is the field-of-view(FOV) of the magnetic resonance image. The resolution is determined bythe ratio of the field-of-view and the number of samples.

The sub sampling may be achieved in that respective receiver antennaeacquire MR signals such that their resolution in k-space is coarser thanrequired for the resolution of the magnetic resonance image. Thesmallest wavenumber sampled, i.e. the minimum step-size in k-space, isincreased while the largest wavenumber sampled is maintained. Hence, Theimage resolution remains the same when applying sub-sampling, while theminimum k-space step increases, i.e. the FOV decreases. The sub-samplingmay be achieved by reduction of the sample density in k-space, forinstance by skipping lines in the scanning of k-space so that lines ink-space are scanned which are more widely separated than required forthe resolution of the magnetic resonance image. The sub-sampling may beachieved by reducing the field-of-view while maintaining the largestk-value so that the number of sampled points is accordingly reduced.Owing to the reduced field-of-view sampled data contain contributionsfrom several positions in the object being imaged.

Notably, when receiver coil images are reconstructed from sub-sampledMR-signals from respective receiver coils, such receiver coil imagescontain aliasing artefacts caused by the reduced field-of-view. From thereceiver coil images and the sensitivity profiles the contributions inindividual positions of the receiver coil images from differentpositions in the image are disentangled and the magnetic resonance imageis reconstructed. This MR-imaging method is known as such under theacronym SENSE-method. This SENSE-method is discussed in more detail inU.S. Pat. No. 6,326,786.

Alternatively, the sub-sampled MR-signals may be combined into combinedMR-signals which provide sampling of k-space corresponding to the fullfield-of-view. In particular, according to the so-called SMASH-methodsub-sampled MR-signals approximate low-order spherical harmonics whichare combined according to the sensitivity profiles. The SMASH-method isknown as such from U.S. Pat. No. 5,910,728.

Sub-sampling may also be carried-out spatially. In that case the spatialresolution of the MR-signals is less than the resolution of the magneticresonance image and MR-signals corresponding to a full resolution of themagnetic resonance image are formed on the basis of the sensitivityprofile. Spatial sub-sampling is in particular achieved in thatMR-signals in separate signal channels, e.g. from individual receivercoils, form a combination of contributions from several portions of theobject. Such portions are for example simultaneously excited slices.Often the MR-signals in each signal channel form linear combinations ofcontributions from several portions, e.g. slices. This linearcombination involves the sensitivity profile associated with the signalchannels, i.e. of the receiver coils. Thus, the MR-signals of therespective signal channels and the MR-signals of respective portions(slices) are related by a sensitivity matrix which represents weights ofthe contribution of several portions of the object in the respectivesignal channels due to the sensitivity profile. By inversion of thesensitivity matrix, MR-signals pertaining to respective portions of theobject are derived. In particular MR-signals from respective slices arederived and magnetic resonance images of these slices are reconstructed.

The invention also relates to a magnetic resonance imaging system. It isan object of the invention to provide a magnetic resonance imagingsystem for carrying out the magnetic resonance imaging methods accordingto the invention. The functions of a magnetic resonance imaging systemaccording to the invention are preferably cried out by means of asuitably programmed computer or (micro)processor or by means of aspecial purpose processor provided with integrated electronic oropto-electronic circuits especially designed for the execution of one ormore of the magnetic resonance imaging methods according to theinvention.

The invention also relates to a computer program with instructions forexecuting a magnetic resonance imaging method. It is a further object ofthe invention to provide a computer program whereby one or more of themagnetic resonance imaging methods according to the invention can becarried out. When such a computer program according to the invention isloaded into the computer of a magnetic resonance imaging system, themagnetic resonance imaging system will be capable of executing one ormote magnetic resonance imaging methods according to the invention. Forexample, a magnetic resonance imaging system according to the inventionis a magnetic resonance imaging system whose computer is loaded with acomputer program according to the invention. Such a computer program canbe stored on a cater such as a CD-ROM. The computer program is thenloaded into the computer by reading the computer program from thecarrier, for example by means of a CD-ROM player, and by storing thecomputer program in the memory of the computer of the magnetic resonanceimaging system.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention may take form in various components and arrangements ofcomponents, and in various steps and arrangements of steps. The drawingsare only for purposes of illustrating the preferred embodiments and arenot to be construed as limiting the invention.

FIG. 1. Comparison of sshEPI-DWIs acquired with SENSE (A,C: reductionfactor=3.0) and without SENSE (B,D: no reduction). Arrows indicatesusceptibility artefacts on images without SENSE. Pairs of images (A-B,C-D) have exactly the same slice location.

FIG. 2. A: High SNR 3 T FA map with region of interest (whiterectangle). B: Vector map of the zoomed ROI, overlaid onto an FA map.

FIG. 3. A: 3D-rendering of one subject's brain, depicting forceps minor,a white matter tract through corpus callosum. B: Isosurface rendering ofwhite matter showing the same fibre tract as in A (arrow).

FIG. 4: Images at TE=5 ms with (a) conventional spiral, (b) spiralSENSE. (c) Gradient echo image for reference.

FIG. 5: Images at TE=35 ms. (a) conventional (b) SENSE

FIG. 6: Left: High-resolution sshEPI fMRI using SENSE. Activated pixelsare coloured white. Right: Conventional sshEPI fMRI. Note improvedspatial detail and reduced distortion in the high resolution data.Bottom row: enlarged detail

FIG. 7: Relative signal time courses averaged over activated area.Right: High-resolution SENSE sshEPI Left: Conventional sshEPI. Verticalbars indicate the stimulation periods.

FIG. 8 shows diagrammatically a magnetic resonance imaging system inwhich the invention is used.

DETAILED DESCRIPTION

In a preferred implementation of the present invention,sensitivity-encoding (SENSE) was used for enhancing the feasibility ofhigh-resolution diffusion tensor imaging (DTI) at 3 Tesla. Reducing theecho train length of single-shot EPI (sshEPI), the technique mitigatessusceptibility effects, addressing the key problem of DTI at high field.Based on diffusion tensor maps from healthy volunteers, fibre trackingin the frontal white matter was performed and visualised with a newlydeveloped software package.

The study shows that SENSE permits robust single-shot DTI at 3 Tesla.The signal-to-noise ratio (SNR) benefit of high field is thus madeavailable for fibre tracking with high spatial resolution.

Introduction

Diffusion tensor imaging (DTI) using single-shot EPI (sshEPI) enablesnon-invasive tracking of cortical white matter fibres in the humanbrain. Critical shortcomings of sshEPI are image blurring and itssensitivity to field inhomogeneity, especially at high field strengths.However, the SNR benefit of high field could considerably enhance DTIand fibre tracking.

Recently, the potential of the parallel imaging technique SENSE wasdemonstrated in combination with diffusion-weighted MRI using sshEPI at1.5 Tesla. SENSE was shown to significantly reduce artefacts byshortening the EPI train. Hence, in this study we explore SENSE-DTI at 3Tesla, making the enhanced SNR of high field available for fibretracking in the human brain.

Resulting white matter tracts were displayed using a newly developedvisualisation tool, which enables observing embedded tracts fromdifferent view-angles and viewpoints using a transparent 3D context.

Methods

Whole brain datasets of three healthy volunteers were acquired on a 3 TPhilips Intera whole body system (Philips Medical Systems, Best, theNetherlands). For all SENSE acquisitions, an 8-element head coil arrayMRI Devices Corporation, Waukesha, USA) was utilised. 3D gradient echoimages, acquired interleaved from the body coil and the SENSE coil array(matrix/α/TE=34□34/7°/17 ms/4.6 ms), served as references forsensitivity calculations; Diffusion-weighted SENSE-sshEPI scans (FOV=200mm, 38 slices, thickness=3 mm, TR>6 s, phase enc.=AP) with SENSEreduction factors of 1.0, 2.0, 2.5 and 3.0 (TE=105 ms, 89 ms, 79 ms and75 ms) were carried out along six directions (−2/3−1/30−2/3)^(T)(1/3−2/3)^(T) (−2/3 2/3 1/3)^(T) 1/√2(1 1 0)^(T) 1/√2(0−1−1)^(T) 1/√2(10−1)^(T) with a maximal b-factor of 1000 s/mm². In order to limit T2decay, partial Fourier encoding of 75% was applied. AfterSENSE-reconstruction each slice matrix consisted of 128□103 points withan actual resolution of 1.6×1.9×3 mm³. After zero filling a resultingresolution of 1.6×1.6×3 mm³ was used for DTI calculations and fibrereconstruction. A total of 16 averages (4 scans, each≈3′30″) wereacquired to enhance SNR. Subsequently, a rigid interscan registrationwas applied and eddy-current-induced image warping was removed with anon-rigid registration algorithm. The eigenvalues and eigenvectors weredetermined by singular value decomposition and fibre tracking wasperformed using an improved version of the FACT algorithm. Resultingfibre tracts were visualised in a 3D environment with a newly developedsoftware package taking advantage of OpenGL.

Results

FIG. 1 clearly shows the benefit of SENSE-sshEPI at 3 Tesla on imagequality. Susceptibility-related distortion artefacts and image blurringare significantly reduced. The unfolding procedure of SENSE had nonegative implication on any of the diffusion weighted images (DWIs).Fractional anisotropy (FA) maps (FIG. 2A) demonstrate the high SNR athigh field even for a slice thickness of 3 mm and a relatively shorttotal scan time (˜14 min). The map's contour illustrates successfulimage registration. In FIG. 2B, a vector field map is shown, revealingdetailed structure of tracts in the right frontal white matter. FIG. 3depicts in two different representations the reconstructed forcepsminor, a white matter fibre tract through the corpus callosum,connecting the frontal lobes of the two hemispheres.

Discussion

It has been shown that high resolution DTI at 3 Tesla is madeconsiderably more feasible by parallel imaging. It diminishessusceptibility artefacts and image blurring, preserving the advantagesof sshEPI, such as motion robustness. The high SNR at 3 Tesla and theexcellent image quality of SENSE-DTI permit precise white matter fibretracking, as demonstrated here for a single bundle. The SENSE-DTItechnique together with the presented visualisation package offers atool to investigate the connectivity between different brain areas,providing new knowledge to the understanding of neuronal networks.

Time efficiency in single-shot spiral MRI was increased by usingsensitivity encoding (SENSE) for parallel data acquisition. Utilizing asix-element receiver array the spiral readout duration was reduced by afactor of two without altering demands on gradient performance.Iterative SENSE reconstruction yielded images free of aliasingartefacts. The technique was applied to functional BOLD MRI with visual,motor, and taste stimulation and compared with conventional spiralacquisition. The SENSE results showed considerably reduced artefacts inbrain regions with high susceptibility gradients, which partly enabledrecovery of activation in these areas. SNR and fMRI stability in SENSEwere reduced by 20% and 13%, respectively.

Introduction

Spiral readout techniques are frequently used in functional brain MRI(fMRI) based on blood-oxygen-level-dependent (BOLD) contrast due totheir superior motion and flow properties compared with Cartesiantrajectories However, spiral acquisition is prone to artefacts andsignal loss due to dephasing caused by strong susceptibility fieldgradients. This effect is particularly pronounced in single-shot imagingwith long readout periods.

A means of improving time efficiency in MRI is parallel data acquisitionwith multiple receiver coils such as sensitivity encoding (SENSE).Recently, an iterative algorithm has been presented enabling efficientreconstruction of data from arbitrary trajectories. In a preferredimplementation of this invention, SENSE was used to reduce the readoutduration in single-shot spiral imaging. The technique was applied toBOLD fMRI. Image quality and statistical stability were compared toconventional spiral acquisition.

Methods

Single-shot spiral imaging was performed using a square field-of-view of240 mm and a matrix size of 80×80. The readout duration of theconventional spiral trajectory was 36 ms using a maximum gradientstrength of 18 mT/m and a maximum slew rate of 99 mT/m/ms. For SENSEimaging a reduction factor of R=2 was applied by reducing the samplingdensity in the radial direction while keeping it constant along thetrajectory. This enabled a shortening of the acquisition window to 18 mswithout altering the demands on the gradient peak values. The wholebrain was covered with 24 axial slices of 5 mm thickness acquired every2.5 s. Spectral-spatial excitation was applied with a flip angle of 90°.Images were acquired with a TE of 5 ms as well as 35 ms in thefunctional studies.

The experiments were carried out at 1.5 T on a Philips Gyroscan Interausing a receiver coil array with 6 rectangular elements of 10×20 cm².The coils were arranged around the head without overlapping adjacentelements.

Sensitivity-encoded spiral data were reconstructed using the iterativegridding approach described. The reconstruction converged after at mostfour iterations. The conventional spiral data were reconstructed withstandard gridding but optimal complex combination of the data frommultiple coils and image intensity correction. fMRI was performed ineight healthy volunteers using either visual, motor, or tastestimulation.

FIG. 4 shows images obtained with (a) conventional spiral and (b) spiralSENSE imaging acquired at TE=5 ms. In FIG. 4 a signal loss andoff-resonance blurring occur in regions where strong susceptibilitygradients are present, in particular in the orbito-frontal region. WithSENSE (FIG. 4 b) these effects are reduced considerably due to theshorter readout period. Furthermore, less blur-ring occurs from fatsignal, edges are sharper, and the spatial resolution is improved due toless T2* decay. Images with TE=35 ms are shown in FIG. 5. Again theSENSE results (FIG. 2 b) are superior to conventional spiral imaging(FIG. 5 a). Analysis of the fMRI data of all subjects yielded a ratio ofSENSE versus conventional spiral for the activation volume of 1.01×0.86whereas in some cases SENSE enabled recovery of activation in theorbito-frontal cortex. The respective ratios for SNR and SFNR were0.80±0.08 and 0.87±0.10.

Sensitivity encoding enables reduction of susceptibility artefacts insingle-shot spiral imaging. Even though the benefit is stronger at shortTE, images sensitised to BOLD contrast also show improved quality. Thereduction of SNR and fMRI stability is less severe than by thetheoretical value of √2 associated with the shorter acquisition time, asadditional signal loss occurs in the second half of the conventionalreadout. Suitable applications in fMRI are studies focused on brainregions with strong susceptibility gradients such as the orbito-frontalcortex. A combination with reversed spiral acquisition and specificexcitation pulses is promising. In this invention a single-shot EPI(sshEPI) sequence was combined with the parallel imaging technique SENSEin order to demonstrate fMRI with very high spatial resolution at 3Tesla. Using an array of six receiver coils and a SENSE reduction factorof 2.7, a full 192×192 image matrix per shot was acquired, yielding aneffective in-plane resolution of 1.0×1.0 mm².

The notorious problem of strong susceptibility gradients at 3 Tesla wasthus mitigated both by enhanced encoding efficiency and reducedintra-voxel dephasing per unit time. As a result, motor activation wasdepicted with greatly enhanced spatial accuracy.

Introduction

The number of phase encoding steps in sshEPI is limited by variousfactors, most importantly by gradient performance and susceptibilityeffects. In the context of BOLD fMRI additional constraints are theprescribed optimal echo time and limited contrast-to-noise ratio (CNR).As a consequence, the spatial resolution of fMRI studies using sshEPI isusually relatively low.

In order to improve spatial resolution in fMRI, we propose combining theCNR benefit of high field strength with the parallel imaging techniqueSENSE. Reducing the echo train length of sshEPI, SENSE mitigatesgradient and susceptibility issues and thus permits pushing spatialresolution beyond current limitations. In this invention we demonstratehigh-resolution SENSE fMRI at 3 Tesla using a typical motor blockparadigm.

Methods

Data Acquisition: Measurements were performed in a healthy volunteer ona Philips Intera 3.0T whole body MR unit (Philips Medical Systems, Best,The Netherlands) equipped with a TR body coil and a commercial 8-elementhead receiver array (MRI Devices Corporation, Waukesha Wisc., USA).Functional data were obtained from 9 transverse slices containingprimary motor areas with a spatial resolution of 1.0×1.0×5 mm³ (matrixsize 192×192) using SENSE-sshEPI with a reduction factor of 2.7, 80%partial Fourier, TE=35 ms, and TR=2000 ms. For comparison the experimentwas repeated with conventional sshEPI with otherwise the same parametersand the largest image matrix achievable at preserved TE, i.e. 112×112,corresponding to 1.6×1.6×5 mm³ resolution.

Stimulus and Paradigm: Stimulation consisted of bilateral oppositefinger tapping in four 20 s-on/off-periods.

Postprocessing: Data were motion corrected and a linear drift correctionwas applied to the signal time courses. A statistical threshold ofp<0.0001 was chosen to hig

Results

The resulting BOLD activation maps are displayed in FIG. 6, overlaid tosshEPI images taken from the dynamic series. The improved detail of thehigh resolution image is readily appreciated. It also exhibits slightlyreduced susceptibility artefact as compared with the conventional image.Both experiments yielded activation focused in the motor area, with theleft-right bias stemming mainly from a slight tilt of the head. There isexcellent agreement between the activated regions in the two maps.However, due to higher resolution the activation in the SENSE casereflects the underlying gyri more accurately. The total activated areais 16% smaller in the high resolution map, most probably due to lesspartial volume effect. Signal time courses averaged over the activatedarea are displayed in FIG. 7 illustrating that the CNR was stillsufficent for correlation analysis at high resolution. The percentsignal change was even somewhat higher in the SENSE experiment.

Discussion

This study demonstrates that the speed benefit of parallel imagingpermits trading the SNR advantage of high field for higher resolution.CNR permitting, improved spatial resolution is of great interestespecially for advanced MRI studies, focusing on details of corticalfunctional topography.

Susceptibility effects, which increase with field strength, are reducednot only by SENSE acquisition but also by reducing voxel size. As aresult, the high resolution data presented exhibit less susceptibilityartefact than the conventional data Slight reduction in the activatedarea is probably related to reduced partial volume effect. Partialvolume effect may also partly underlie the increase in percent signalchange observed in the high resolution experiment. However, this effectrequires more detailed investigation, as it may as well be related topixels with low activation, which are detected only in the lowresolution case with higher CNR, lowering the average activation there.

FIG. 8 shows diagrammatically a magnetic resonance imaging system inwhich the invention is used.

The magnetic resonance imaging system includes a set of main coils 10whereby the steady, uniform magnetic field is generated. The main coilsare constructed, for example in such a manner that they enclose atunnel-shaped examination space. The patient to be examined is slid intothis tunnel-shaped examination space. The magnetic resonance imagingsystem also includes a number of gradient coils 11, 12 whereby magneticfields exhibiting spatial variations, notably in the form of temporarygradients in individual directions, are generated so as to be superposedon the uniform magnetic field. The gradient coils 11, 12 are connectedto a controllable power supply unit 21. The gradient coils 11, 12 areenergized by application of an electric current by means of the powersupply unit 21. The strength, direction and duration of the gradientsare controlled by control of the power supply unit. The magneticresonance imaging system also includes transmission and receiving coils13, 16 for generating the RF excitation pulses and for picking up themagnetic resonance signals, respectively. The transmission coil 13 ispreferably constructed as a body coil whereby (a part of) the object tobe examined can be enclosed. The body coil is usually arranged in themagnetic resonance imaging system in such a manner that the patient 30to be examined, being arranged in the magnetic resonance imaging system,is enclosed by the body coil 13. The body coil 13 acts as a transmissionaerial for the transmission of the RF excitation pulses and RFrefocusing pulses. Preferably, the body coil 13 involves a spatiallyuniform intensity distribution of the transmitted RF pulses. Thereceiving coils 16 are preferably surface coils 16 which are arranged onor near the body of the patient 30 to be examined. Such surface coils 16have a high sensitivity for the reception of magnetic resonance signalswhich is also spatially inhomogeneous. This means that individualsurface coils 16 if are mainly sensitive for magnetic resonance signalsoriginating from separate directions, i.e. from separate parts in spaceof the body of the patient to be examined. The coil sensitivity profilerepresents the spatial sensitivity of the set of surface coils. Thereceiving coils, notably surface coils, are connected to a demodulator24 and the received magnetic resonance signals (MS) are demodulated bymeans of the demodulator 24. The demodulated magnetic resonance signals(DMS) are applied to a reconstruction unit. The reconstruction unitreconstructs the magnetic resonance image from the demodulated magneticresonance signals (DMS) and on the basis of the coil sensitivity profileof the set of surface coils. The coil sensitivity profile has beenmeasured in advance and is stored, for example electronically, in amemory unit which is included in the reconstruction unit. Thereconstruction unit derives one or more image signals from thedemodulated magnetic resonance signals (DMS), which image signalsrepresent one or more, possibly successive magnetic resonance images.This means that the signal levels of the image signal of such a magneticresonance image represent the brightness values of the relevant magneticresonance image. The reconstruction unit 25 in practice is preferablyconstructed as a digital image processing unit 25 which is programmed soas to reconstruct the magnetic resonance image from the demodulatedmagnetic resonance signals and on the basis of the coil sensitivityprofile. The digital image processing unit 25 is notably programmed soas to execute the reconstruction in conformity with the so-called SENSEtechnique or the so-called SMASH technique. The image signal from thereconstruction unit is applied to a monitor 26 so that the monitor candisplay the image information of the magnetic resonance image (images).It is also possible to store the image signal in a buffer unit 27 whileawaiting further processing, for example printing in the form of a hardcopy.

In order to form a magnetic resonance image or a series of successivemagnetic resonance images of the patient to be examined, the body of thepatient is exposed to the magnetic field prevailing in the examinationspace. The steady, uniform magnetic field, i.e. the main field, orientsa small excess number of the spins in the body of the patient to beexamined in the direction of the main field. This generates a (small)net macroscopic magnetization in the body. These spins are, for examplenuclear spins such as of the hydrogen nuclei (protons), but electronspins may also be concerned. The magnetization is locally influenced byapplication of the gradient fields. For example, the gradient coils 12apply a selection gradient in order to select a more or less thin sliceof the body. Subsequently, the transmission coils apply the RFexcitation pulse to the examination space in which the part to be imagedof the patient to be examined is situated. The RF excitation pulseexcites the spins in the selected slice, i.e. the net magnetization thenperforms a precessional motion about the direction of the main field.During this operation those spins are excited which have a Larmorfrequency within the frequency band of the RF excitation pulse in themain field. However, it is also very well possible to excite the spinsin a part of the body which is much larger than such a thin slice; forexample, the spins can be excited in a three-dimensional part whichextends substantially in three directions in the body. After the RFexcitation, the spins slowly return to their initial state and themacroscopic magnetization returns to its (thermal) state of equilibrium.The relaxing spins then emit magnetic resonance signals. Because of theapplication of a read-out gradient and a phase encoding gradient, themagnetic resonance signals have a plurality of frequency componentswhich encode the spatial positions in, for example the selected slice.The k space is scanned by the magnetic resonance signals by applicationof the read-out gradients and the phase encoding gradients. According tothe invention, the application of notably the phase encoding gradientsresults in the sub-sampling of the k space, relative to a predeterminedspatial resolution of the magnetic resonance image. For example, anumber of lines which is too small for the predetermined resolution ofthe magnetic resonance image, for example only half the number of lines,is scanned in the k space.

The invention has been described with reference to the preferredembodiments. Modifications and alterations may occur to others uponreading and understanding the preceding detailed description. It isblended that the invention be constructed as including all suchmodifications and alterations insofar as they come within the scope ofthe appended claims or the equivalents thereof.

1. A magnetic resonance imaging method comprising: applying a staticmain magnetic field with a main field strength; applying an acquisitionsequence with an RF-excitation which generates an echo train ofsuccessive magnetic resonance signals from an object to be examined, theacquisition sequence including a diffusion sensitization subsequence;receiving the magnetic resonance signals with a degree of undersamplingby means of a receiver antennae system having a spatial sensitivityprofile, the degree of undersampling is set on the basis of an amount ofphase evolution due to a magnetic susceptibility distribution of theobject to be examined; reconstructing a magnetic resonance image fromthe magnetic resonance signals and the spatial sensitivity profile;repeatedly applying the excitation sequence, receiving the magneticresonance signals and reconstructing the magnetic resonance image toproduce a series of magnetic resonance images; matching said magneticresonance images and determining mutual matches between said magneticresonance images; selecting a number of the magnetic resonance imagesfrom the series on the basis of the mutual matches; averaging over saidselected number magnetic resonance images.
 2. A magnetic resonanceimaging method as claimed in claim 1, wherein the degree ofundersampling is dependent on the main field strength.
 3. A computerprocessor or media programmed to perform the steps of: repeatedlyapplying an excitation sequence, receiving resultant magnetic resonancesignals, and reconstructing the magnetic resonance signals to produce aseries of magnetic resonance images; comparing the magnetic resonanceimages of the series to determine images with at least a preselecteddegree of similarity; combining the magnetic resonance images of theseries which have at least the preselected degree of similarity.
 4. Amagnetic resonance imaging system including a main magnet which appliesa static magnetic field, an RF coil, a transmitter for applying anacquisition sequence with an RF excitation which generates an echo trainof successive magnetic resonance signals from an object to be examined,a receiver antenna system having a spatial sensitivity profile, areceiver for receiving magnetic resonance signals from the receiverantenna system, and the computer processor media as claimed in claim 3.5. A magnetic resonance imaging system including a main magnet whichapplies a static magnetic field, an RF coil, a transmitter for applyingan acquisition sequence with an RF excitation which generates an echotrain of successive magnetic resonance signals from an object to beexamined, a receiver antenna system having a spatial sensitivityprofile, a receiver for receiving magnetic resonance signals with adegree of undersampling from the receiver antenna system, and acontroller, the controller being programmed to: selectively adjust thedegree of undersampling in accordance with an amount of phase evolutionartifacting attributable to a magnetic susceptibility distribution ofthe object to be examined.
 6. The apparatus as claimed in claim 5further including a reconstruction processor programmed to: repeatedlyapply the excitation sequence, receive the magnetic resonance signals,and reconstruct the magnetic resonance signals to produce a series ofmagnetic resonance images; compare the magnetic resonance images of theseries to determine images with at least a preselected degree ofsimilarity; combine the magnetic resonance images of the series whichhave at least the preselected degree of similarity.